Generally, there are known a hammer drill, a vibration drill, and a diamond drill as the kind of concrete drill tool. A hammer drill or a vibration drill subjects concrete to impact fracture to drill by rotating a nearly solid drill bit attached with an ultra-hard tip at a front end thereof while exerting a striking force or a vibration force in an axial direction. Therefore, though a drilling speed is fast, extremely large noise is emitted in operation by the striking force or the vibration force for subjecting the concrete to the impact fracture. Further, since impact by the striking force and the impulsive force is directly propagated to the concrete, the impact is easily propagated from the concrete at which drill operation is carried out to concrete forming other wall portions or floor portions of a structure. In result, the impact is propagated to a concrete wall and a concrete floor in a room remote from the place at which the drill operation is carried out to emit large noise in a wide range of the structure.
Meanwhile, according to a diamond drill, a front end of a bit in a nearly cylindrical shape or in a nearly columnar shape is attached with a diamond tip in which diamond particles are embedded in a sintered metal referred to as metal bond. By applying pressing force and rotation to the diamond bit, the diamonds of the diamond tip at the front end of the bit bite the concrete, and the concrete is ground, whereby drilling is carried out. The size of a single particle of the diamond of the diamond tip is about 400 micrometers, and a single piece of the bit includes about 1500 particles of the diamond particles. The fine diamond particles cut concrete to drill. Therefore, in the diamond drill, small sound is emitted in drill operation, and the striking force and the impulsive force are not propagated to the concrete not as in the hammer drill and the vibration drill. Therefore, at a room remote from the place at which the drilling operation is carried out even, the sound during drilling operation is considerably low, which makes execution of work possible while people are living in the same structure or at a neighboring house.
In order to accelerate the drilling speed of the diamond drill, it is necessary to increase the concrete cutting amount of the diamond. As means of increasing the concrete cutting amount, there are thought to increase the number of rotations of the diamond bit thereby to increase the cutting amount per time, or to increase the pressing force for pressing the diamond bit, that is, a tool body to a concrete face and to increase the diamond biting amount into the concrete thereby to increase the cutting amount. However, it is important to balance the number of rotations of the diamond bit with the pressing force. In case that the number of rotations of the diamond bit is increased in a state where the pressing force is insufficient, the diamond bit rotates in a state where the front end of the diamond bit cannot bite the concrete sufficiently. In result, the diamond bit wears out early and drilling becomes difficult. Further, the pressing force of the diamond bit (tool) which the operator can keep exhibiting in operation is generally about 10 to 15 Kgf. Accordingly, the operator has a limit in increasing the pressing force of the diamond bit (tool) on the basis of only his bodily powers. In result, even in case that the operator tries to accelerate the drilling speed of the diamond drill, there is a limit. In addition, in case that the operator tries to bore a hole having a larger hole diameter under a condition where there is a limit to the pressing force which the operator can exhibit, as the hole diameter becomes larger, the pressing force per diamond particle decreases. Therefore, it is difficult to accelerate the drilling speed also from the viewpoint.
In order to solve such the problem, in JP-A-2003-211436, a concrete drill has been disclosed, in which a vibrating apparatus for exerting a vibrating force which acts in an axial direction of a bit drive shaft, a magnitude of which pulsates, is provided for a concrete drill apparatus body thereby to accelerate the drilling speed for concrete.
Under constitution of the concrete drill in JP-A-2003-211436, the vibrating force which pulsates along the bit drive shaft is simply exerted. Therefore, from the drilling starting time, the vibrating force is exerted. In result, when drilling starts, the front end of the diamond bit leaps up from the concrete face which is the drilling place due to the vibrating force, and it can become difficult to align the bit front end with the drilling place exactly. Therefore, there is a problem that operability is bad.